1. Field of the Invention
This invention relates to an image formation method and apparatus, and more particularly to an image formation method in which light information such as light signal or light image is applied to a photosensitive member and an image corresponding to the light information is formed on the photosensitive member by conductive particles, and an apparatus to which such method is applied.
2. Description of the Prior Art
As a method for forming a toner image on the light portion of a photosensitive member, a method comprising uniformly pre-charging the photosensitive member, and then applying an image light to the photosensitive member to thereby form an electrostatic latent image thereon, thereafter causing toner particles to adhere to the photosensitive member by a developing device correspondingly to the pattern of the electrostatic latent image, thereby obtaining a visible image has been widely used (this will hereinafter be referred to as the first method). In this case, it is usual to employ insulative particles as the toner particles and use them by pre-charging them to the same polarity as the polarity of the latent image on the photosensitive member due to the friction between the insulative particles and carrier particles or the like.
It is known that it is also effective to apply to the developing device a DC bias voltage of the same polarity as the polarity of the latent image on the photosensitive member at this time. Also, in Japanese Laid-open Patent Application No. 4532/1974 (corresponding U.S. Pat. No. 3,909,258), it is disclosed to effect the development by using conductive and magnetic particles as the toner.
Where the first method is used to form a toner image on the light portion of the image light on the photosensitive member, a DC bias voltage of substantially the same value as the potential of the toner carrier of the developing device and the dark portion of the image light on the photosensitive member is applied to the developing device so that in the portion corresponding to the dark portion, no potential difference is created between the developing device and the photosensitive member but only in the portion corresponding to the light portion, a potential difference is created between the photosensitive member and the toner carrier of the developing device, whereby toner is caused to adhere to the portion corresponding to the light portion.
However, where the first method is used, the steps of uniformly charging the photosensitive member by a corona charger or the like, applying the image light, and causing the toner to adhere by the developing device must be effected in succession, and an apparatus for realizing this becomes not only bulky and complex but also often defective and inferior in reliability. In particular, the charging means for uniformly charging the photosensitive member requires a high voltage source having an output of several kilovolts and is not only bulky and expensive but also involves the danger of electric shock. It is also susceptible to the influence of humidity, powder, etc. and inferior in reliability.
On the other hand, what is called the smoke printing method is known as a method which does not use the step of charging the photosensitive member and in which the step of applying the image light and the step of developing are carried out at a time (this will hereinafter be referred to as the second method). This method comprises using a photosensitive member having a photoconductive layer provided on a transparent and conductive substrate, placing a metal net-like electrode in opposed relation with the photoconductive layer, applying a light image through the transparent and conductive substrate and simultaneously therewith, blowing toner from the metal net-like electrode side, and causing the toner to be adsorbed onto a support member such as paper provided in advance in intimate contact with the photoconductive layer. As the photoconductive layer, use is made of CdS which is an N type semiconductor and in this case, a minus voltage is applied to the net-like electrode side.
This second method can obtain an image by a very simple process, but it suffers from much fog produced in the background of the image and with this method, it is difficult to obtain toner images of high image density and thus, this method has not yet been put into practical use.
Japanese Patent Publication No. 48821/1973 (corresponding U.S. Pat. No. 3,563,734 and DEOS No. 1,797,187) discloses that a conductive pattern formed on a photosensitive member is developed by a developer having conductivity and magnetism to thereby form an image (this will hereinafter be referred to as the third method). In this method, development is effected while applying a DC voltage between a conductive substrate on the back of the conductive pattern formed by applying a light image to the photosensitive member and the toner carrier of the developing device. An insulative layer is provided between the conductive substrate and the photoconductor.
The formation of the conductive pattern may be effected prior to or simultaneously with the development.
In the third method, where toner is caused to adhere correspondingly to the dark portion of the image light (where a positive image is to be obtained), if the photoconductor is an N type semiconductor, a negative voltage is applied to the substrate of the photoconductor and, if the photoconductor is a P type semiconductor, a positive voltage is applied to the substrate of the photoconductor. Where toner is caused to adhere correspondingly to the light portion of the image light (where a negative image is to be obtained), if the photoconductor is an N type semiconductor, a positive voltage is applied to the substrate of the photoconductor and, if the photoconductor is a P type semiconductor, a negative voltage is applied to the substrate of the photoconductor. In this case, the applied voltage is of the order of 500 V and should not be a mere DC voltage but must be a pulsating DC electric field.
If the third method is employed, a negative image can be obtained without using charging means, but where this negative image is to be obtained, an exposure amount of two to three times is necessary as compared with the case where a positive image is to be obtained, and an exposure amount of ten times or more is required as compared with the case where, as is usually done, the photosensitive member is charged and image exposure is provided to thereby form an electrostatic latent image. In the example disclosed in the aforementioned Japanese Patent Publication No. 43821/1973, use is made of an exposure amount of about 400 lux seconds and this is about 20 to 200 times the exposure amount required in the presently commonly used electrophotographic copying apparatuses. Accordingly, it is difficult to speed up the development and usually, a time of two to three seconds is required for the formation of a negative image. Also, the source of the applied voltage must be a pulsating DC power source as previously mentioned and this makes it difficult to employ a flashlight, a laser light modulated by image electrical signals, etc. as the source of image light. This is because, where exposure is effected by a source of light such as the flashlight or the modulated laser light which provides an exposure during a very short time, the image density fluctuates and a stripe pattern is created in the image depending on whether the voltage of said pulsating DC power source is in its high condition or in its low condition at the moment of exposure.
As described above, the image formation methods of the prior art have their own merits and demerits, and a method which can form a negative image for a relatively small exposure amount and at a high speed without using a complicated step such as the charging step and can form an image by scanning and exposing by the use of a modulated laser light has not yet been known.
In recent years, it has been widely practised to scan a photosensitive member by a laser light or the like modulated by image electrical signals and provide exposure to thereby obtain an image, and in this case, it is known that a more beautiful image can be obtained if the portion corresponding to the black of the image is exposed to form an image.
Also, a semiconductor laser is often used as the laser, but the light emitted by a semiconductor laser usually is near-infrared light which is low in sensitivity to a photoconductor, and the energy obtained therefrom is of the order of 10 mW at best. An image formation method effective for these usages has not yet been realized although it has been strongly desired.